With regard to a Ge-MOSFET which is expected to be a next-generation device, application of a germanide, such as NiGe, to a metal contact to a source/drain (S/D) or metal S/D has been studied. The germanide is advantageous in that an S/D resistance is lower than that in n+-Ge or p+-Ge formed by high-concentration doping of an impurity and that the germanide can be formed on a Ge layer in a self-aligning manner.
However, in a case of forming a germanide by deposition of a metal on a Ge layer and annealing of the metal and Ge, the germanide is polycrystalline and disadvantageous in that an interface has a low degree of flatness. In the case of Ge nMOSFET with metal S/D, a contact resistance between the germanide S/D and an n-type (or p-type) Ge is increased (or reduced), accordingly, an on (off) current is reduced (or increased).
The reason for this may be that a Fermi level of a germanide is pinned to a charge neutral level near a valence band of Ge (Fermi level pinning: FLP). With increasing of the interface states, the FLP becomes stronger. Therefore, an interface having a high degree of flatness is desirable to lower interface state.